Freeze plugs (also called "blind" or "expansion core" plugs) are small steel plugs used to seal the holes in the engine block and head made in casting. They expand and flatten as they are driven into place, and make a tight seal. These are designed to hold the pressure of the cooling system, but to pop out if the coolant in the block ever freezes. If you have a leak in your cooling system, freeze plugs are one of the areas to have checked.